Infection of humans by any micro-organisms leads to the initiation of a humoral immune response that can be used in the diagnosis of the disease. In the early acute phase of the infection, specific IgM class antibodies are the first to appear in serum 1-4 weeks after the onset of symptoms and last for up to three months. IgG class antibodies appear later and remain elevated throughout the patient's life. Detection of an IgM response is indicative of a recent or current infection, while the presence of an elevated IgG response is a marker for past exposure to the causative agent. Specific IgM or IgG responses to a particular infectious agent can be measured by antibody based diagnostic tests such as ELISA, immunochromatography, particle agglutination ELISA, biosensor or other similar assays.
These assays require the use of reactive human sera as a positive control. The positive control reagent is usually serum taken from a patient or animal which is known to have a positive reaction to the particular antigen under test. If the test is designed to distinguish between early and late infection (via the differentiation between immunoreactive IgM, for early infection and IgG, for late or previous infection), the positive control serum or reagent should contain immunoreactive antibody of the correct immunoglobulin class.
It is becoming increasingly difficult to source sufficient quantities of immune human sera or plasma, particularly as diagnostic tests for rarer diseases become available. Collection of blood for IgM controls from patients in early stages of infection when clinical symptoms are generally most severe poses significant ethical problems, particularly if the disease primarily affects juveniles. Other drawbacks include the requirement for consistent collections from remote locations, the need to standardise each batch and to check for contamination with infectious agents such as HIV, hepatitis B and hepatitis C. There are also problems in obtaining control sera for specific endemic diseases in communities where the donation of blood or blood products is socially unacceptable.
There is therefore a need for a source of positive control reagents which does not rely on being obtained from human donors.
Hybridoma technology provides a plentiful supply of monoclonal antibodies, but as these are generally of murine origin, they do react with binding reagents used to quantify human antibodies. Intact functional mouse/human chimeric antibodies have been described in the literature for some time (Boulianne et al., 1984, Morrison et al., 1984; Winter et al., 1991). In these constructs the antigen binding function residing in a mouse Fab or Fv fragment has been grafted on to a human Ig backbone and expressed in hybridoma cells. In some cases these reshaped molecules have been designed for human therapy, utilising the effector functions of the human Fc for targeting (Reichmann et al., 1988). Others have been designed as positive control reagent substitutes (Hamilton, 1990, 1991), where VH and VL regions from a mouse monoclonal antibody of desired specificity have been grafted onto either a human IgG or IgM backbone.
Synthetic positive control reagents are available from a limited number of sources. U.S. Pat. No. 4,929,543 relates to chimeric antibody fragments where Fab or F(ab′)2 fragments of non human origin, with specificity for the desired antigen, are chemically coupled to human Fc fragments in order to confer upon the reactive non-human Fab fragments epitopes recognised by class specific anti human immunoglobulin antisera. This reference does not teach or suggest coupling non-human Fab or F(ab′)2 fragments to individual CH domains in order to provide epitopes for recognition by class specific anti human immunoglobulin antisera. Furthermore, production of the chimeric fragments is entirely by synthetic routes based upon digestion of antibodies, purification of fragments and chemical linking to create the chimera.
Labor Diagnostika GmbH of Heiden, Germany have produced synthetic positive control reagents which are formed by chemical attachments of non human Fab fragments and human Fc fragments onto a latex bead. These attachments confer upon the bead the twin properties required of a positive control reagent—specific antigen binding and human immunoglobulin class specific epitopes.
A process for producing positive control reagents which circumvents the requirement to manipulate full length Fc fragments or to manipulate VH and VL sequences for each new control reagent specificity is desirable.